There are two typical optical circulators in the prior arts, including transparent and reflective circulators. The input optical signal incident onto one of the input/output ports of the circulator is split into ordinary and extraordinary rays, and the two rays are converted into output signals respectively and then transmitted onto another desired input/output port. Those incident lights from different ports are transferred to other ports associated such that a sequential order of the input/output ports for transformation of optical signal is constructed and the function of the circulator is achieved.
With reference to FIG. 1, a reflective optical circulator in the prior arts includes two reflective input/output devices 101 and 102, a first front birefringent crystal 12, a second front birefringent crystal 13, a first Faraday polarization rotator 14, a set of back birefringent crystals 15, a second Faraday polarization rotator 16, and a mirror with dielectric coating 17. The set of back birefringent crystals 15 further consists of two crystals 15a and 15b aligned in the X direction. The optic axes of the two crystals 15a and 15b are in the -Y and Y direction respectively, i.e., the crystal 15a is at an upper position if the X direction is defined to be upper as shown in FIG. 1(A). The angle between the optic axis of the first front birefringent crystal 12 and the Y-axis is 45 degrees, and the angle between the optic axis of the second front birefringent crystal 13 and the Y-axis is -45 degrees.
With reference to FIG. 2, the optical points on the circulator in FIG. 1 are shown, where FIG. 2(A) illustrates the respective spot positions and the optic axes of the ordinary and extraordinary rays on each optical component for the conversion from port 1 to port 2, and FIG. 2(B) from port 2 to port 3.
For an incident light onto port 1, the optical signal for the forward path passes through the first front birefringent crystal 12, the second front birefringent crystal 13, the first Faraday rotator 14, the set of back birefringent crystals 15, and the second Faraday rotator 16, and their respective positions and optic polarization states are shown in A-1, A-2, A-3, A-4, A-5, and A-6 respectively.
Subsequently, the ordinary and extraordinary rays are reflected by the mirror 17, and pass through the second Faraday rotator 16, the set of back birefringent crystals 15, the first Faraday rotator 14, and the second front birefringent crystal 13, to the first front birefringent crystal 12. The respective positions and optic polarization states are shown in A-7, A-8, A-9, A-10, and A-11. The two split rays, ordinary and extraordinary, are combined into a single ray, so a complete conversion from port 1 to port 2 is accomplished.
The conversion from port 2 to port 3 is similar to the above-mentioned process from port 1 to port 2. The respective positions and the optic polarization states are shown in B-1, B-2, B-3, B-4, B-5, B-6, B-7, B-8, B-9, B-10, and B-11. Finally, the two rays are combined into a single ray onto port 3 as a resulting output signal. Thus, a complete conversion from port 2 to port 3 is accomplished. The number of optical components in the prior arts are too many and the size of the circulator is relatively large because of employing two front birefringent crystals (12 and 13), and two back crystals (15a and 15b). Therefore, both the size of the circulator and the material cost can not be reduced.